The application field of chargeable and dischargeable secondary batteries is increasingly being expanded to electric vehicles as well as portable devices such as mobile phones, notebooks, and camcorders. Accordingly, secondary batteries have been actively developed. Also, research and development of battery design to improve capacity density and specific energy have been conducted during the development of the secondary batteries.
In general, it is known that battery safety improves in the order of a liquid electrolyte, a gel polymer electrolyte, and a solid polymer electrolyte, but battery performance decreases in the same order.
An electrolyte in a liquid state, particularly, an ion conductive organic liquid electrolyte, in which a salt is dissolved in a non-aqueous organic solvent, has been mainly used as an electrolyte for an electrochemical device, such as a typical battery using an electrochemical reaction and an electric double-layer capacitor. However, when the electrolyte in a liquid state is used, an electrode material may degrade and the organic solvent is likely to be volatilized. Also, there may be limitations in safety such as combustion due to a high ambient temperature and the temperature rise of the battery itself.
In particular, since an electrolyte used in a lithium secondary battery is in a liquid state and may have a risk of flammability in a high-temperature environment, this may impose a significant burden on electric vehicle applications. The above limitations may be addressed when the lithium electrolyte in a liquid state is replaced with a solid-state electrolyte. Thus, various solid electrolytes have been researched and developed to date.
A flame retardant material has been mainly used as a solid electrolyte and as a result, since the solid electrolyte is formed of a highly stable and non-volatile material, the solid electrolyte is stable at high temperature. Also, since the solid electrolyte may act as a separator, a typical separator is not required and a thin film process may be possible.
Among them, since a perovskite-structure oxide having a chemical formula of Li0.33La0.66TiO3 (LLTO) is a material having high chemical stability and durability, a significant amount of research into this material has been conducted.
However, since a typical solid electrolyte may have high interfacial resistance, low ionic conductivity, and low flexibility due to the contact between an electrode and the solid electrolyte, there are various limitations in terms of processing.